Photodiodes are widely used for sensing light radiation. There are many applications in which the level of light which is required to be sensed is very low, and therefore the sensitivity of said photodiodes is a critical requirement.
It is well known in the art that the signal-to-noise ratio which can be obtained from photodiodes (and from many other electronic components) is limited by the level of the “thermal noise”, which in turn is related to the temperature of the component. The term “dark current” is commonly used in the art to define the current flowing in a photodiode during a total dark condition. The signal-to-noise ratio in photodiodes is conventionally improved by cooling the component, in some cases down to very low temperatures close to 0° K. The means for cooling and maintaining such a low temperature in photodiodes, however, are cumbersome and expensive, and in any case can only reduce the noise down to a limited value.
In an ideal photodiode, the dark current is generally composed of two main components. The first component, hereinafter referred to as “the diffusion” dark current, is due to the thermal excitation of carriers across the complete energy band gap of the photon absorbing material of the photodiode, followed by diffusion of minority carriers thus created to the depletion region. The second component affecting the level of the dark current is known as the “Generation-Recombination” current, hereinafter referred to as “G-R” dark current. A reverse bias applied to the diode activates these G-R centers in the depletion region of the photodiode, so that they can provide energy levels close to the middle of the band gap. As a result, the amount of thermal energy or “activation energy” needed to excite an electron (charge carrier) in the depletion region from the valence band to the conduction band is approximately halved compared with the diffusion current process. Moreover, when electron-hole pairs are generated in this way, they are immediately removed by the electric field of the depletion region, thereby providing a strong driving mechanism for the G-R current. While the level of the G-R dark current can also be reduced by cooling, it is reduced at a slower rate than the diffusion current, due to its smaller activation energy. At low temperatures, where the level of the diffusion dark current is reduced sufficiently, the G-R dark current generally becomes the most dominant component of the dark current. There have been made many efforts in trying to reduce the level of the thermal noise. However, there are not known many such efforts for reducing the G-R current.
In a real photodiode, there is often a third component to the dark current which is parasitic in nature. This component is due to an unwanted surface leakage current which acts to shunt the resistance of the depletion region of the device. This current is often relatively insensitive to temperature and thus cannot be totally eliminated by cooling the device. It is usually suppressed by applying an appropriate passivation treatment to the exposed surfaces of the device in the junction region but the effectiveness of this treatment can vary according to the materials used.
In widely used III-V infra-red detector photodiodes based on materials such as InSb, which is well matched to the mid-wave infra-red (MWIR) atmospheric transmission window (approximately 2.9-5.4 μm), the dark current is invariably due to the G-R mechanism. Recently, new types of photodetectors have been demonstrated in which heterostructures are used to increase the band gap of the depletion region at a barrier layer of the photodetector. As a result, the G-R current is strongly suppressed so that the diffusion current becomes dominant. The dark current noise of the detector is then very low. These devices are often referred to as XBn or XBp detectors.
The XBn and XBp photo-detectors relate to a class of infra-red photo-detectors developed as an alternative to traditional photodiode detectors currently incorporated into many infrared detector systems. Four different architectures were proposed for the XBn or XBp photo-detectors in U.S. Pat. Nos. 7,795,640 and 8,004,012 assigned to the assignee of the present application. Detectors of the XBn/XBp detector family include: a photon absorbing layer, a barrier layer, and a contact layer on top of which metal contacts, to be connected to a suitable readout circuit, may be placed. In XBn/XBp detectors, the photon absorbing layer and barrier layer have the same doping polarity (n-type in XBn devices and p-type in XBp devices), so that a depletion layer is not formed at the junction between the barrier layer and the photon absorbing layer (see [08]). In XBn photodetectors, the barrier layer is constrained to have both a negligible valence band energy offset (e.g., less than a few kBTop where kB is Boltzman's constant and Top is the detector operating temperature) and a large conduction band energy offset (e.g., equal to, or larger than, the band gap of the photon absorbing layer) with the photon absorbing layer, and the valence band of the contact layer lies close to or above the valence band of the barrier layer. XBp photodetectors are the polarity reversed version of XBn photodetectors, having also an inverted band structure. Accordingly, in XBn/XBp photodetectors, majority carriers in the contact layer are prevented from entering the barrier layer by a large energy barrier, while at the operating bias of such detectors, minority carriers in the photon absorbing layer (holes in XBn and electrons in XBp) can diffuse freely into the barrier layer, from which they drift into the contact layer under the influence of the electric field in the depleted barrier layer. XBn/XBp devices are generally designed to have no depletion of the photon absorbing layer (e.g., since both the photon absorbing and barrier layers have the same doping polarity) such that G-R currents are suppressed and the main source of dark current is constituted by diffusion current. The dark current in such XBn/XBp devices is therefore lower than the dark current in a conventional photodiode made from the same photon absorbing material. In [07] band structures of the XBn and XBp photodetector devices are described.
Detectors known as nBn/pBp present a particular case of the XBn/XBp photodetector in which the contact layer is made from the same material as the photon absorbing layer and is doped with the same polarity. References [09] and [10], disclose versions of detectors that include a photon absorbing layer, a barrier layer and a contact layer, where the barrier layer is undoped. In the XBn/XBp configurations described above, it is doped with the same doping polarity as the photon absorbing layer. A device/detector with an undoped barrier can often have a background doping in the barrier layer which is opposite to that of the photon absorbing layer (i.e., p-type barrier in XBn and n-type barrier in XBp), and which will form a depletion region sourcing G-R current from the photon absorbing layer and so the dark current from such devices will not be diffusion limited as in the case of the XBn devices described above.
U.S. patent publication 2012/0273838 describes minority carrier based HgCdTe infrared detectors and arrays.
U.S. Pat. No. 4,679,063 proposes an alternative design of barrier structure (hereinafter referred to as an mBn structure) in which the barrier layer has the opposite doping polarity to the photon absorbing layer, and the contact layer is replaced/formed by metal.